1. Field of the Invention
The present invention relates to an information recording and reproducing apparatus, and more particularly to such apparatus adapted for use as an interface for data transfer between units of different data transfer rates.
2. Description of the Prior Art
Conventionally so-called optical and magnetooptical disk apparatus are known as information recording and reproducing apparatus. Among such known apparatus, the magnetooptical disk apparatus, for example, has a structure as shown in FIG. 1.
In FIG. 1, a magnetooptical disk 10 is composed of a magnetic recording layer 18 formed on a disk-shaped transparent substrate 16 composed for example of glass or plastic material. A recording head 26, composed of a light source such as a laser, an optical system such as a collimating lens and an objective lens, external magnetic field generating means, focusing means etc. concentrates a recording light beam 46 from the light source onto the recording layer 18 to elevate temperature locally and simultaneously applies a magnetic field, thereby orienting the magnetization of said local area in the direction of the externally applied magnetic field and thus recording the information. A reproducing head 38 composed of a light source such as a laser, an optical system such as a collimating lens, an objective lens and a polarizer, a photosensor such as a PIN photodiode, focusing means, tracking means, etc. concentrates a reproducing polarized light beam of low power not affecting the magnetization of the recording layer 18 onto a determined track of the recording medium, thus reading the recorded information in the form of magnetization by means of magnetic Kerr effect. More specifically, a linearly polarized light beam (light with an electromagnetic vector vibrating only in one direction) obtained through the polarizer is directed onto the recording layer, and the reflected light, the plane of polarization is rotated according to the direction of magnetization of the recording layer, is guided through an analyzer to convert the rotation of the plane of polarization into a change in the light intensity and is received by a photosensor to reproduce the recorded information.
The magnetooptical disk 10 is rotated by a motor 12, while the recording head 26 and the reproducing head 38 are moved in the radial direction of said disk 10 respectively along rails 20, 34. Consequently the information is recorded in spiral form or concentric form on the magnetooptical disk 10 from the external periphery toward the interior.
In case of data transfer between two units of different data transfer rates, it is necessary to convert hhe data transfer rate. As an example, the data transfer from a unit I of a higher transfer rate R1 to another unit II of a lower transfer rate R2 (R1&gt;R2) is generally conducted in the following manner. At first data are released from the unit I with the transfer rate R1 and are stored in memory means. In the course of or after said storage, the data are reproduced from said memory means at the transfer rate R2 and supplied to the unit II. The data transfer from the unit II to the unit I is also conducted by way of such memory means.
In this manner the mmory means is indispensable for the conversion of the transfer rate, and for this purpose a semiconductor memory such as a random access memory (RAM) has been employed. However such semiconductor memory has been associated with a drawback in that the cost per unit memory capacity is high.
For this reason there is considered the use of a magnetooptical disk apparatus as shown in FIG. 1 for the above-mentioned memory means. For example, in case of data transfer from the unit I of the transfer rate R1 to the unit II of the transfer rate R2 (R1&gt;R2), the magnetooptical disk 10 is rotated at a rotary speed .omega.1, and a data train of a determined length (hereinafter called block data) is supplied from the unit I to form a record on a track 60 from X to Y, as shown in FIG. 2, by means of the recording head 26. Then the angular speed of the magnetooptical disk is changed to .omega.2 (.omega.1&gt;.omega.2) so as to obtain a transfer rate R2 of the reproduced data, and the data from X to Y are reproduced by the reproducing head 38 and supplied to the unit II. Upon completion of the signal reproduction, the angular speed is again changed to .omega.1, and new block data are released from the unit I and recorded anew on the track from X to Y. Subsequently the angular speed is changed again to .omega.2 for signal reproduction. The above-described procedure is repeated until all the required data are transferred from unit I to unit II.
The use of a magnetooptical apparatus for the conversion of the transfer rate allows the cost of the entire system to be reduced because of the large capacity and low cost of the magnetooptical disk. However such system requires a long data transfer time because the conventional magnetooptical disk apparatus requires a change in the rotational speed of the disk between the information recording and reproduction and is therefore unable to conduct the information recording and reproduction at the same time.